Abstract Fourier transform infrared (FTIR) spectroscopy is a rapid, economical, easy, and non-destructive technique. The IR spectrum of a clay mineral is sensitive to its chemical composition, isomorphous substitution, layer stacking order, or structural modifications. This makes FTIR spectroscopy the most informative single technique not only for clay mineral composition and structure but also for interactions of the clay minerals with inorganic or organic compounds. After a brief presentation of the principles of the technique, the chapter gives an overview of the potential of FTIR for mineralogical identification of clay minerals samples, and for studying reactions with water and organic molecules. This chapter includes data from all three infrared regions, that is, far, middle, and near.